The default linux drivers allow this. Quadro NVS 400 boards contain two GPUs and because of this the card appears as two videocards. The card needs to be set up as two videocards acting in Twinview (and grouped together using a ServerLayout section) or you need to set it up as four separate X screens which you can group together by enabling Xinerama.

For more info check the driver docs but also search at this forum as more people had your problem in the past.